In prospecting in subsea and other areas underlying a body of water, it is desirable to provide a source of energy for propagating sonic pulses or shock waves into the water. Since water is a good conductor of sound, it normally is not necessary to generate pulses near the floor of the waterbody; the pulses can be, and desirably are, produced near the water's surface. These pulses propagate down through the water, across the water-floor interface, into the subfloor geologic formations and are, to some extent, reflected back across the same path to an array of hydrophones waiting near the surface of the water. Analysis of the signals produced by the hydrophones can provide information concerning the structure of the subfloor geological formations and attendant petroleum accumulation in those formations.
The term "water" as used herein is meant to include swampwater, mud, marshwater and any other liquid containing sufficient water to enable operation of the invention.
There are many ways of generating a seismic pulse in a liquid. For instance, explosives introduce strong pulses into the water and accordingly achieve substantial penetration into subfloor formations. Certain obvious drawbacks exist in their use: they are dangerous to store, handle, and use. When used in open water they kill marine life. In crowded areas such as harbors, explosives cannot be used at all. Explosives are orders of magnitude more expensive to use, on a per-shot basis, than are air guns. Modification of the explosive source sonic signature to achieve an acceptable spectrum distribution is difficult.
Another method of generating a sonic pulse is by discharge of a bank of capacitors through a subsurface electrode to produce a quickly collapsing gaseous bubble. However, the efficiency of this method is quite low in that only a few percent of the energy charged to the capacitors is found in the shock wave produced on discharge.
Apparatus using explosive gas mixtures, e.g., propane and air, to produce the sonic pulse have gained wide acceptance. The two major types of explosive gas guns are those which operate by exploding a gas mixture behind a flexible membrane which in turn is in contact with the water and those which operate by allowing the abrupt bubble from the gas explosion to pass directly into the water. An example of the former apparatus can be found in U.S. Pat. No. 3,658,149; an example of the latter can be found in U.S. Pat. No. 4,193,472.
Open air guns using high pressure compressed air, instead of an explosive mixture, have achieved a wide acceptance in the industry. Typical designs for open-ported compressed air guns are found in U.S. Pat. No. 3,653,460 to Chelminski and U.S. Pat. No. 4,141,431 to Baird. These guns employ two pressurized chambers, i.e., a control chamber and an air-holding chamber, which are sealed by a spool-shaped valve or shuttle. The gun is fired by abruptly releasing air from the control chamber. The air in the air-holding chamber forces the shuttle into the control chamber and thereby simultaneously exposes the exhaust ports. These ports allow the air stored in the air-holding chamber to exit explosively into the water. The control chamber is then repressurized, thereby moving the shuttle back into a position sealing the air-holding chamber. The gun is again ready to "fire."
Guns employing this design have certain liabilities which are quite difficult to correct. The period during which the exhaust ports are open after escape of the initial burst of pressurized air is one in which no useful operation is performed. The initial burst of air through the exhaust port is the one which produces the useful portion of the shock wave. Obviously the air lost from the exhaust ports during the repositioning of the shuttle is wasted. The apparatus of the invention disclosed herein consumes a significantly smaller amount of compressed air than do those of the prior art having a shuttle which must reverse itself prior to firing. The mechanical stress on the shuttle of the invention is much less than on the reversing shuttle of the prior art.
A gun which uses compressed air and eliminates several problems associated with prior guns is disclosed in U.S. Pat. No. 4,180,139 to Walker, issued in Dec., 1979, U.S. Pat. No. 4,211,300 to Miller, issued in July 8, 1980, and U.S. Pat. No. 4,324,311, to Farris, issued Apr. 13, 1982.These patents disclose a gun having a single cylindrical air chamber with central exhaust ports about its periphery. Inside the air chamber resides a moveable shuttle also having ports about its center. When the shuttle is moved from one end of the air chamber to the other end, via the action of an integrated actuator, the ports in the shuttle momentarily align with those in the air chamber wall and allow an amount of compressed air to escape. Once the shuttle reaches the other end, the gun is in position to "fire" again awaiting only the build-up of pressure in the air chamber and actuating mechanism. Although this device has a number of advantages over those of the prior art, e.g., efficient compressed air usage, the geometry of the device precludes any reasonably facile alteration in the frequency and spectrum distribution of the sonic pulse it produces. The two physical dimensions of the gun, i.e., exhaust port geometry and air chamber size, defining the signature of the sonic pulse cannot be changed in the field without substantial effort.
A seismic source using a single shuttle to seal two air storage cylinders and subsequently release the compressed air contained in those cylinders is shown in U.S. Pat. No. 4,381,044 to Kirby, issued on Apr. 26, 1983.